I know I seen a while back a setup where you can push a button and a y-pipe opens up so you can run open header. Well I know it works good for V8s that produce lots of torque but will running open header improve a hondas time in the quarter mile (b18a1 with all bolt ons no internals)?

 

depends on how restrictive the exhaust is...

the principle behind it is that backpressure from a restrictive exhaust impedes exhaust flow, and hence, can rob SOME hp at top end, when your engine is pushing out lots of exhaust.

there is also the approach that a little backpressure at low-midrange RPM helps pull exhaust out of the cylinders, letting more unburnt air come in (called scavenging).

however, on a N/A B18A, 60-70mm piping should be a pretty nice balance for optimal performance... so, if u have an exhaust on there already, provided its around a 2-2.5" pipe, you should be fine. if its stock, you will want to go bigger, and if its open, you will get a ticket driving there

bottom line is: open header = most top end power, but usually a slight loss in low-mid range response. but a very restrictive exhaust will usually give you about the most low end torque you can get, at the expense of top end power (and keep in mind the gains and losses here are very minimal)

 

Well I have 2.5 inch exhaust from ractive with airmass header. The setup I was looking at cuts open the exhaust from a push of a button (its electronic). What I was thinking is that since my car is N/A I would start off the line with the exhaust like normal and once I am able to shift into second or third I would push the button to run open header this way I can try and get the best of both worlds. I dunno just an idea I was tossing around.

 

with a 2.5" exhaust you will be fine, it wont be worth the hassle, and i bet it would rattle around and sound crappy on the street.

plus, exhaust flow acts as a fluid, and and fluid dynamics book will tell you that you only really need the open-ness when the time rate of change of exhaust flow is highest ( i.e. going to redline in first, maybe second gear) if you open it just for third, (this all depends on gearing, im assuming an LS trans) your RPMs are increasing slowly enough that flow is essentially uniform, and hence, you wont notice much of a change in speed during slow acceleration pulls.

it all has to do with the engine having to change the velocity of the exhaust gas through the pipes, and first and second gear (and third, depending on gearing) are the only times you will really notice the difference.... once the engine isnt changing the exhaust velocity (dV/dt => 0) the size of the pipe is almost negligible, as the entire pipe is filled with exhaust gas at a near uniform velocity.

bottom line: with the open header/cutout thingy, you may gain 3-7 hp top at peak power (6200 RPM ish), but you may also lose 2-5 ft-lbs where the torque curve comes up (4000 RPM ish).

this is the classic debate of what size piping you should use... if a cutout/open header made the most power, and it was USEABLE, everyone would be putting 5 inch exhaust pipes on their cars. but with the scavenging effects of smaller piping, its much more efficient to use midsized pipe for optimal performance.

the same reason all the aftermarket N/A exhausts (tanabe, apex'i, greddy, etc) use around a 60-65mm pipe for all their performance exhausts.

this could go on indefinitely tho, and im not the best at describing this stuff, so ima stop there. hope it helps!

 

Yea I deff. understand what your saying and I guess the only time something like a cut out would be useful is forced induction due to the high pressure. Thanks for saving me money lol.

 

In my experience in my first race ever at bradenton and on take off when I blew my cat clean off the header thus running open header but not by choice, I to this point ran my best time and my honda went from being the most quiet to the loudest. They may not like you running open header at the track at night, they don't wanna wake their neighbors.

 

the theroy of back pressure is a load of sh!t (for racing). if your racing, the less back pressure the better.

~ so for your question, is open header better for the 1/4 mile.....yes

 

ive got a good power gain off of 2-1/4 straight piping to the rear wheel off my header.